Referring to FIG. 1 herein, conventional inkjet printer devices, especially of the type for printing on B size media format, or of the large format type, comprise a media transport mechanism 100 for carrying a sheet of print media 101, the media transport mechanism comprising a set of rollers, a set of control motors for controlling the rollers, and a set of guides for guiding the media, and a print head carriage 103. The carriage comprises a print head having a plurality of inkjet nozzles. Typically, the carriage traverses across the print media in a direction transverse to a direction of movement of the print media through the print mechanism.
With current inkjet printer technology, pen variability can lead to variations in print quality. To achieve a successful print quality, pen variability needs to be compensated for. Calibration in order to compensate for pen variability is known as the automatic alignment process. One of the purposes of the automatic alignment process is to rectify the angle of misalignment which can occur between an image printed onto a print media, and the boundaries of a print media. This angle is know as theta zeta, and is introduced by defects in the printing system, comprising the pen, carriage and print media. The objective is to assure that the drops of ink deposited by a print head onto a media are placed onto a perfect straight and vertical line.
A basic assumption is made that the inkjet nozzles are correctly aligned on the pen. The main defects in the printing system arise from defects in positioning between the pen, the carriage which carries the pen, and the print media. The inkjet nozzles naturally print on a straight line which is nominally vertical. An object of calibration is to make the straight line vertical with respect to the print media. Therefore, the angle between a nominally vertical line printed by the pen and a main vertical axis of the paper needs to be measured.
As a prior art calibration process, estimation of the angle theta zeta consists of printing a set of patterns onto a print media, and then scanning them, and applying an algorithm to compare the actual geometry of the pattern with a theoretical geometry of the pattern. The differences between the theoretical positions of the pattern and the scanned positions of the pattern are characteristic of the defects in alignment which are to be corrected.
Each group of nozzles prints a line of squares. A first line of squares is printed by an upper part of the pen, and so on down to a lower part of the pen. The pattern is scanned in line by line. By locating all the squares produced by a pen, the angle of the pen relative to the paper axis can be calculated.
Referring to FIG. 2. Herein, there is illustrated schematically a printed pattern comprising an array of squares, which is printed by a pen, and then scanned back in to the printer device.
An algorithm is applied in order to determine the angle of the pen relative to the main axis of the print media.
However, several constraints make the performance of this algorithm poorer than the performance which could be expected. One of the constraints is the skew in the paper introduced when the media advances between consecutive scans of the pen across the print media. In fact, what is actually measured with the algorithm is the angle between a nominal ‘vertical’ line as printed by the pen during the print phase, and the movement performed by the media during the scan phase. To properly determine the angle of misalignment, theta zeta, there needs to be determined how many degrees are due to the skew of the print media, and how many degrees are due to the defect which is to be corrected. Therefore, the amount of skew needs to be measured.
Referring to FIG. 3 herein, there is illustrated schematically a rectangular sheet of media 300 having an image 301 printed thereon. In a printer device in which the pens and carriage are perfectly aligned, relative to the transport mechanism for the media, the image can still be slightly skewed relative to the print media, due to misalignment of the print media within the media transport mechanism. An angle between a main length axis of the image and main length axis of the print media is know as the ‘skew angle’ and is illustrated schematically in FIG. 3. The skew angle could equally be defined as an angle between a main width axis of the printed image and a main width axis of the print media.
Referring to FIG. 4. herein, there is illustrated schematically a pattern of squares printed onto a print media. A currently known method for measuring skew is to evaluate a mean position of the squares of each line across a print media which is scanned. This gives a ‘mean point’, for each line of the printed pattern.
For each row of squares, there is a mean position denoted ‘X’. An overall mean position line 200 can be determined from the mean points of each individual row of the pattern. In a perfectly aligned print system, the mean points would lie on the same vertical line relative to the print media. However, in practice, due to defects in the print system, the points may lie on a line which forms an angle to true vertical relative to the print media. The angle between the line of mean points and true vertical is equal to the skew angle. Once the skew angle is determined, this can be used to refine the evaluation of the angle theta zeta.
Referring to FIG. 5. herein, there is illustrated schematically basic process steps carried out by a prior art algorithm for determining the skew angle from a printed pattern of squares. In step 500, the mean position of each row of squares is evaluated. This gives the mean position of each row 501. In step 502, there is constructed a best fit line passing between the mean position of each row of squares. In step 501, there is determined an angle between this best fit line, and a true vertical line, which is taken as the skew angle 503.
However, the above method for determining skew angle proves to be poorly accurate when applied to mechanical printer devices. The theta zeta correction performance is lowered by the rough evaluation of the skew angle.